1. Field of the Invention
The present invention relates to a semiconductor ceramic having a negative temperature coefficient of resistance and a negative temperature coefficient thermistor.
2. Description of the Related Art
In recent years, more accurate negative temperature coefficient thermistors, which are mainly used as temperature sensors, have been required. It has also been required that the variation of resistance be controlled to be within plus or minus one percent. Conventionally, spinel composite oxides made of a solid solution of Mn and at least one element from the group Zn, Mg, Al, and transition elements (Ti, V, Cr, Fe, Co, Ni, Cu) excluding Mn have been used as semiconductor ceramics used in such negative temperature coefficient thermistors. However, it is generally known that the composite oxide creates a problem of environmental resistance. It is believed that the problem is caused by Mn ions changing their state of oxidation and migrating between sites in accordance with changes in the environmental temperature and the partial pressure of oxygen.
In order to solve this problem, research has been conducted, and in a paper by B. Gillot et al. (Solid State Ionics, 48, 93-99, 1991) and a paper by A. Rousset (Journal of the European Ceramic Society, 13, 185-95, 1994), a method wherein barium is added when raw materials are input is reported. According to these papers, because barium has an ionic radius which is larger than the ionic radius of the transition elements, barium is not solid soluble in a spinel phase and exists in grain boundaries and at a triple point by forming a different phase. Because such a construction is formed, changes in resistance are greatly suppressed in high temperature environments of 125xc2x0 C.
Furthermore, in Japanese Examined Patent Application Publication No. 6-48641, it is stated that changes in resistance are controlled in a high-temperature environment at 125xc2x0 C. by adding an oxide of a rare earth element or oxides of aluminum and a rare earth element to a thermistor element made of oxides of Mn and Ni.
However, according to the methods described in the above-mentioned papers and patent application publication, because free water-soluble barium ions are likely to remain in the raw material and sinter, gelation of binders takes place deteriorating the moldability, and the oxide of unreacted rare earth elements becomes likely to remain. As a result, swelling of molded bodies occurs due moisture absorption and new problems of performance in highly humid environments are occur. These facts were made clear by the experiments conducted by the present inventors and others.
Therefore, it is an object of the present invention to provide a semiconductor ceramic having a negative temperature coefficient of resistance and a negative temperature coefficient thermistor which are good in moldability and highly reliable under high humidity environments.
In order to achieve the above object, in a semiconductor ceramic having a negative temperature coefficient of resistance according to the present invention, about 0.1 to 20 mol % of AMnO3 (A represents at least one of Ca, Sr, Ba, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy and Ho) is added to a spinel composite oxide comprising a solid solution of Mn and at least one Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Mg and Al element.
Furthermore, at least a pair of electrodes is provided on the surface of or inside an element assembly comprising the semiconductor ceramic in a. negative temperature coefficient thermistor according to the present invention.
Regarding the addition of Ca, Sr, Ba, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho at the time when the semiconductor ceramic is produced, because free water-soluble ions and the oxide of rare earth elements are reduced after firing and sintering by selection of a perovskite Mn composite oxide, a negative temperature coefficient thermistor of a small variation in characteristics, in which the swelling of the molded body caused by reaction of binders and water absorption is suppressed and the reliability under high humidity environments is excellent, can be obtained.
According to the present invention, CaMnO3, SrMnO3, BaMnO3, LaMnO3, PrMnO3, NdMnO3, SmMnO3, EuMnO3, GdMnO3, TbMnO3, DyMnO3 and HoMnO3 may be used as the perovskite Mn composite oxide. One of these may be used or two or more may be used together.
The reason why the addition of AMnO3 is limited to the range of about 0.1 to 20 mol % is that, when less than about 0.1 mol % is added, the effect of the addition cannot be recognized and when more than about 20 mol % is added, the resistance value and the B constant become too large. Furthermore, there is greater change in resistance under high humidity environments.